George Bernard Shaw's political writing had a profound impact on me as a teenager, and four decades later I am conscious of a personal debt to him. I still retain the beard I first grew in a spirit of emulation, even if it became a miniaturised version of his luxuriant growth when an election agent instructed me to cut it on the maxim, "More hair: fewer votes".

Shaw's distinctive contribution to the Fabian ethos was to humanise it. He parted from Marx over the determinist character of the doctrine of historical inevitability, which wrote free will out of the script as effectively as Calvinism. For Shaw, socialism could only be achieved by human will and conscious effort. His key commitment was to the power of reason, and he therefore believed that socialism must be built by persuasion, by argument and by deliberate choice.

The consequence was that he became the most active proselytiser among the early Fabians. In the decade after the formation of the Fabian Society, he addressed 1,000 meetings - in trade councils, Working Men's Clubs and debating societies, in every environment from open-air parks to smoke-filled bars. Many of his earliest plays were written on trains and trams travelling to and from these engagements.

The motivation for this extraordinary campaigning output was in part a powerful anger at the injustice of Victorian society, but in part also a dramatist's enjoyment of performance. By all accounts the combination of his intellect and sense of drama was compelling. Annie Besant, the prototype feminist, was engaged to debate against Shaw but after hearing his speech, rose to announce her conversion to Fabianism.

Perhaps one of the aspects of Shaw that attracted her was his stout and outspoken advocacy of women's rights. His plays are replete with confident, assertive women who share his rejection of the injustice of "saddling the right to a child with the obligation to become a servant of a man". Emmeline Pankhurst confided that the character of Ann Whitefield in Man and Superman had "strengthened her purpose and fortified her courage". In his last political work, Shaw asserted that Parliament could only properly be called a House of Commons when men and women were represented in equal numbers, a goal that half a century later still eludes us.

Another Shavian theme that speaks to our time is his disgust at the stupidity and barbarity of war. Shaw was one of the first to point out that "the mechanisation of modern war greatly reduces the power of human conscience" and that as a result, a youth of ordinary good nature "will release a bomb that will blow a whole street of family homes into smithereens, burning, blinding, mutilating scores of mothers and babies, without seeing anything of his handiwork". If he were alive today, Shaw would have exposed the moral contrast between our emotional response to the appalling decapitation of a single person with a knife and the relative indifference of our media to the killing of 10,000 Iraqis by bomb and missile.

Shaw's polemic on the folly of the British government in participating in the first world war was so magnificent that it immediately made him the target of patriots on the home front. His books were removed from libraries, his plays from London's West End and the son of prime minister Asquith called for him to be shot. True to its long tradition of liberal vacillation, the Guardian refused to publish a letter from him because "one's duty now is to encourage and unite people". Shaw's response to these critics was an uncompromising call to the soldiers of both armies to "shoot their officers and go home".

It was supreme confidence in his intellectual superiority that sustained Shaw in persisting in his views against all comers. This was a source of strength that made him an iconoclastic figure, enthusiastic about challenging convention and orthodoxy in a way that would rapidly have caused him to be expelled from New Labour. The quintessential Shaw quotation is the one which celebrates the revolutionary power of original thought: "The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man."

Yet over-confidence can also be a source of weakness. Shaw's fixation on the pioneering role of the intellectual contributed to his unhealthy tendency to attach too much importance to heroic leadership. This led him into the trap of naive admiration for the top-down modernisation of Russia under the leadership of the Bolsheviks, which is a cause of embarrassment to even the most devoted Shavian. Its sole redeeming feature is that Stalin left their two-hour meeting complaining that Shaw was an awful person.

Fortunately for Fabianism, Shaw had a much better grasp from first-hand knowledge of the condition of working people in Britain. The result was a body of drama and tracts that to this day is unrivalled for its acute and biting account of an economic system that exposed the majority of the population to ruthless exploitation. Ironically, it is not the intellectual analysis of which Shaw was so proud that makes his writing still resonate with us today, but his powerful moral indignation. In Mrs Warren's Profession, prostitution is only a metaphor for the experience of a proletarian class exploited by a bourgeoisie which owed their respectable place in society to a financial system that to Shaw was equally immoral.

Nor has Fabianism ever boasted another pamphleteer of such mordant wit. Shaw was a contemporary of Oscar Wilde and although his output had a more serious purpose, he could produce repartee just as sharp. On his 90th birthday he was interviewed by a young journalist who took his leave by expressing the hope that he would be able to interview Shaw when he was 100. With characteristic self-confidence Shaw replied: "I don't see why not. You look healthy enough."

· This article appears in Fabian Thinkers: 120 years of progressive thought , published by the Fabian Society at £6.95 on October 20. www.fabian-society.org.uk.